This invention relates to implantible articles and methods for implanting such articles. More particularly, the invention relates to bone prostheses and methods for implanting the same.
There are known to exist many designs for and methods for manufacturing implantible articles, such as bone prosthesis. Such bone prostheses include components of artificial joints, such as elbows, hips, knees and shoulders. An important consideration in the design and manufacture of virtually any implantible bone prosthesis is that the bone prosthesis has adequate fixation when implanted within the body.
Early designs of implantible articles have relied upon the use of cements such as polymethylmethacrylate (PMMA) to anchor the implant. The use of such cements can have some advantages, such as providing a fixation that does not develop free play or does not lead to erosion of adjoining faces post-operatively. Maintaining a load or force at the cement/bone interface assists in providing for good fixation and to prevent motion.
One challenge in the proper positioning of the prosthesis during surgery is the proper position of the stem axially and rotationally. Improper positioning has been shown to limit the patient's range of motion by inducing improper leg length, inadequate lateral stem offset and non-anatomical version of the stem. Inadequate pressurization of the cement within the femoral canal has also been documented as a potential cause of improper cement technique.
Centralization of the stem within the cement mantel is also critical for success. Non-uniform or excessively thin cement mantels can induce high cement stress and subsequent cracks that cause failure at the cement-stem-bone interfaces. The cement debris, due to abrasions, has also been shown to produce excessive third-body wear of polyethylene acetabular components as well as potentially induce osteolytic reactions and bone resorbtions that may lead to stem loosening.
One devise utilized to assist in the centralization of the stem is the use of centralizers or spacers. Centralizers or spacers are provided for fitting to the distal end of a femoral hip replacement stem in order to keep the implant stem away from the internal surface of the cavity of the bone in which this stem is to be inserted.
In the case of stems which are cemented in the bone cavity there is a space between the stem and the internal surface of the cavity of the bone in which the cement is placed. Controlling the position of the stem within the surrounding bone cement mantle is vital to long-term survivability of the replacement joint. Cement can be deposited in the bone cavity and then the stem may be inserted with the centralizer attached to the stem. Alternatively, the centralizer may be inserted into the cavity and the stem later inserted against the centralizer. It is important to try to obtain an even and intact cement mantle around the stem.
In addition to the purpose of the centralizer to properly position the stem, the centralizer may be designed to serve a second purpose, that is to separate the cement from the blood and other body fluids within the medullary canal of the bone. Such separation of cement and medullary canal fluids is exasperated by the more recent use of external pressure to assure the complete filling of the bone cavity with cement.
Known centralizers are in the form of caps which fit over the distal end of the stem and centralizers which are fixed inside of a drilled end of a stem. Centralizers are also known, for example, as described in U.S. Pat. No. 4,658,351 which are of ring form which can have a tapered inner surface corresponding to the tapered surface of the distal stem of the femoral stem on which the centralizer is located.
U.S. Pat. No. 3,793,650 describes a centralizer or spacer which has spring members which extend from the stem for contact with the wall of the bone cavity.
European Patent No. EP0427448B describes a centralizer or spacer in the form of a cap for insertion on the end of a hip stem with fins or wings extending outwardly from the cap which are adapted to fold circumferentially and inwardly toward the body portion of the cap.
Cemented stem systems generally utilize two components distal to the stem, a centralizer and a cement plug. The centralizers usually have fins that protrude into the cement mantle around the stem. During insertion, voids often develop around these fins and these voids are potential sites for crack initiation. The distal tip of the stem is subjected to high levels of stress and therefore voids are of a particular concern. The cement plug's only purpose is to restrict the flow of cement during cement pressurization.
Some designs have attempted to optimize the fin geometry by streamlining it in an effort to reduce the number and size of voids. These designs have failed to eliminate voids entirely.
Other designs have shifted the position of the centralizer to the middle of the stem which experiences lower stress levels. This type of design does not eliminate the voids and since centralization of the stem is not controlled distally, the stem positioning may be compromised.